This invention relates to crystalline molecular sieves containing both aluminum and gallium, and is particularly concerned with a method for synthesizing a crystalline gallioaluminosilicate molecular sieve having the faujasite structure.
Zeolites are well known natural and synthetic molecular sieves that can be defined as crystalline, three-dimensional aluminosilicates consisting essentially of alumina and silica tetrahedra which interlock to form discrete polyhedra. The polyhedra are interconnected to form a framework which encloses cavities or voids interconnected by channels or pores. The size of the cavities and pores will vary depending on the framework structure of the particular zeolite. Normally, the cavities are large enough to accommodate water molecules and large cations which have considerable freedom of movement, thereby permitting sorption, reversible dehydration and ion exchange. The dimensions of the cavities and pores in a zeolite are limited to a small number of values and can vary from structure to structure. Thus, a particular zeolite is capable of sorbing molecules of certain dimensions while rejecting those of dimensions larger than the pore size associated with the zeolite structure. Because of this property zeolites are commonly used as molecular sieves.
In addition to their molecular sieving properties, zeolites show a pronounced selectivity toward polar molecules and molecules with high quadrupole moments. This is due to the ionic nature of the crystals which gives rise to a high nonuniform electric field within the micropores of the zeolite. Molecules which can interact energetically with this field, such as polar or quadrupolar molecules, are therefore sorbed more strongly than nonpolar molecules. This selectivity toward polar molecules is the unique property of zeolites which allows them to be used as drying agents and selective sorbents.
In addition to their use as drying agents and selective sorbents, zeolites are widely used as components of chemical conversion catalysts. As found in nature or as synthesized, zeolites are typically inactive because they lack acid sites. In general, acid sites are created by subjecting the zeolite to an ion exchange with ammonium ions followed by some type of thermal treatment which creates acid sites by decomposing the ammonium ions into gaseous ammonia and protons. Activated zeolites have been used in many types of chemical conversion processes with the smaller pore zeolites being used to selectively sorb and crack normal and moderately branched chain paraffins.
Because of the unique properties of zeolitic molecular sieves, there have been many attempts at synthesizing new molecular sieves by either substituting an element for the aluminum or silicon present in zeolitic molecular sieves or adding another element in addition to the aluminum and silicon. The term "zeolitic" as used herein refers to molecular sieves whose frameworks are formed of substantially only silicon and aluminum in tetrahedral coordination with oxygen. One such class of new molecular sieves that has been created is that in which a portion of the framework aluminum has been replaced by gallium. Specifically, it has been reported in U.S. Pat. No. 3,431,219 that gallioaluminosilicate molecular sieves having the faujasite structure have been synthesized. The synthesis process, as illustrated in Example 3 of the patent, involves the separate preparation of a colloidal silica solution and a solution comprised of a mixture of gallium hydroxide dissolved in aqueous sodium hydroxide and sodium aluminate dissolved in water. According to the example, the silica solution was added to the other solution with rapid stirring and the resultant gel maintained at room temperature overnight and then heated to 200.degree. F. Although some crystalline material was present after 6 days, crystallization was not complete until 12 days had past. Such long crystallization times are impractical, and it is therefore clear that a simple process utilizing inexpensive reactants to more rapidly produce gallioaluminosilicate molecular sieves with the faujasite structure is desirable.
Accordingly, it is one of the objects of the present invention to provide a relatively simple and rapid process for synthesizing crystalline, gallioaluminosilicate molecular sieves having the faujasite structure, which sieves may be useful in many types of chemical conversion processes, particularly hydrocarbon conversion processes. This and other objects of the invention will become more apparent in view of the following description of the invention.